Electrical space discharge device



y 1940- P. 1.. SPENCER ET AL 2,206,271

ELECTRICAL SPACE DISCHARGE DEVICE 3 Sheets-Sheet 1 Original Filed June5, 1934 Lmmm) Y Jill/anions July 2, 1940. P. 1.. SPENCER El ALELECTRICAL SPACE DISCHARGE DEVICE Original Filed June 5, 1954 aSheets-Sheet 2 vs fiwenivns' Percgyb Spencer aure MarshaZZ e/iiiafir zegJuly 2, 1940. P. 1.. SPENCER El AL ELECTRICAL SPACE DISCHARGE DEVICEOriginal Filed June 5, 1934 3 Sheets-Sheet 5 PeflqqL Jivenaer PatentedJuly 2, 1940 UNiTED STATES PATENT orrics Marshall,

Cambridge, Mass.,

assignors to Raytheon Manufacturing Company, Newton, Masa, a corporationof Delaware Application 27 Claim.

This invention relates to electrical space discharge devices capable ofcarrying high values of current.

One of the objects of. the present invention is to produce a cathode forsuch a device having a low voltage drop and a long life.

Another object is to enable the device to be started safely after littleor no preliminary heating of the cathode.

Still another object is to produce a device accomplishing the aboveobject, having a simple and rugged construction.

The foregoing and other objects of the invention will be best understoodfrom the following description of. an exemplification thereof, referencebeing had to the accompanying diagrammatic drawings, wherein:

Fig. 1 is a vertical elevation on a reduced scale of the novel tube,with the envelope broken away, said tube shown connected to one type ofcircuit in which it may be used;

Fig. 2 is a vertical cross-section of the lower part of the tube shownin Fig. 1 and particularly of the novel cathode shown approximately fullsize;

Fig. 3 is a bottom view of the lower end of the structure as shown inFig. 2;

Fig. 4 is a top view of the tube as shown in Fig. 1;

Fig. 5 is a top view of one of. the anode assem blies shownapproximately full size;

Fig. 6 is a cross-section taken along line 5-6 of Fig. 5;

Fig. 'l is a vertical cross-section of the upper part of the tube shownin Fig. 1 approximately full size;

Fig. 8 is a cross-section taken along line 8-8 of Fig. 7;

Fig. 9 is a cross-section taken along line 99 of Fig. 6; and

- Fig. 10 is a cross-section of the anode structure taken along line|0I0 of Fig. 5.

Space discharge devices of. the type having a thermionic cathodeoperating with a filling of some suitable vapor require specialprecautions in their starting and operating. Such devices designed tocarry fairly large amounts of current heretofore have utilized a cathodehaving a compar'atively large surface coated with electronemittingoxides, and provided with a separate heating filament. In such devicesprecautions have to be taken to prevent the current density on thecathode surface from rising above a certain value, and also to preventthe cathode from getting too hot at any portion of its surface. If,

June 5, 1934, Serial No. 729,101 Renewed May 8, 1939 for example, thecurrent is allowed to increase beyond the normal currentdensity so as toapproach concentrated arc intensities, the coating is soon disintegratedand removed from the cathode surface. The disintegration of the coatingproduces deleterious gases, and the denuding of the cathode surface ofits coating soon terminates the useful life of the device. Thepermissible current density on the cathode depends to some degree on thevapor pressure around the cathode, and the distribution of the currentover the cathode surface depends to a large degree on the fact whetheror not the entire emitting surface is above a predetermined minimumtemperature. Thus in starting, the heating current for the cathode hasto be turned on for a long period so as to make sure that the entirecoated emitting surface of the cathode is above the required minimum,and also to insure that the entire tube is warmed to the propertemperature so as to maintain the requisite vapor pressure within thedevice. These precautions necessitate a long preliminary heating periodin discharge tubes of any considerable current rating. Also during theoperation of. the device, the current through the device cannot bepermitted to rise above a predetermined maximum even for an instantinasmuch as such a current surge would immediately destroy a portion ofthe coating, which in turn would soon terminate the useful life of thecathode.

In accordance with the invention, a cathode has been constructed whichcan be operated without such coatings as were heretofore used, andfurthermore which can be subjected to enormous overloads for shortperiods of time without adversely affecting the operation of the tube.With such a cathode it has been found that only a limited portion of thecathode need be heated to its starting temperature, and the vaporpressure within the device need not be very great in order that the tubebe safely started. Thus the heating current is merely turned on for ashort period after which the tube is ready to start. If upon startingthe current localizes as an arc upon a limited portion of our cathode,the adjacent portions of the cathode are quickly heated to temperatureof electron emission and the current distributes itself over theoperating surfaces of the cathode. Also if the vapor pressure is notgreat enough to cause a low voltage drop, the heat liberated by thecathode drop soon raises the vapor pressure to the proper value. As hasbeen stated, localized arcs of great current intensity can occur onlimited portions of my cathode without adversely affecting the operationthereof, and thus in starting or running, the localization of the areupon the limited heated portion of the cathode for a short period ispermissible.

The form of the invention as shown in the drawings is a rectifierconsisting of. an evacuated envelope l or" some suitable material suchas glass. This envelope contains a cathode structure 2 and one or moreanode structures The cathode structure consists of a primary cathode 6and a secondary cathode 5. The cathode structure 2 is supported from areentrant stem [i in an en tension "2 formed in the lower side of theenvelope ll. Through the center of the stem 5 is sealed a cathodelead-in conductor This lead-in con= ductor may be made of some suitablematerial, such as tungsten, and the glass of the stem may be made of aborosilicate glass, such as Pyrex.

The primary cathode 4 consists of a hollow tubular member 9 containing aheating filament iii. The member S1 is surrounded by a tubular memberll, which is connected to the upper end of the tubular member a by meansof an annular closing member H2. The members 9, it and i2 provide ahollow cathode chamber 03. The lower end of the tubular member 9 isclosed by a cap it, and has a tubular extension l5 connected to itsupper end. The connection between the extension is and the tubularmember 9 may be made by a sleeve l6 welded to each of these members. Theupper edge or" the sleeve it forms a stop for the insulating plug llwhich fits into the upper end of the tubular extension 65. Theinsulating plug lll is provided with two longitudinal holes throughwhich extend the oon= ductors l8 and is which form the current sup plyleads for the heating filament lit. This heating filament may be formedin the shape of a double helix, the two ends of which are connected tothe lower ends of the conductors iii and is. During the operation of thedevice the tubular member 9 becomes hot, and therefore should be formedof a refractory material, such as tantalum, tungsten or molybdenum. Theannular members l2, l4 and Hi likewise should be formed of a similarrefractory material. The member l i being subjected to a somewhat lowertemper= ature during operation than the member El, may be formed of asomewhat less refractory material, although such materials as mentionedabove are preferably used. The heating filament ill may be of tantalum,tungsten or the like, while the conductors iii and is may be ofmolybdenum The insulating plug ll may be of some suitable insulatingmaterial, such as magnesia.

The secondary cathode 5 consists of a cylindrical member 20 closed atits lower end by a cap it, thus forming a hollow cathode chamber 2d.Both members Zll and El may be formed of nickel inasmuch as thetemperature to which they are subjected during operation of the deviceis considerably less than that to which the primary cathode issubjected. The cap M is provided with a central opening through whichthe upper end of a tubular conduit 22 extends. These two members may berigidly connected together as by welding. The tubular conduit 22likewise may be formed of a refractory material, such. as molybdenum.The secondary cathode is also provided in its hollow cathode chamber 20with a set of fins 23 also formed of nickel and welded in place withinthe tubular member 20. These fins may be spaced radially within thecylindrical member 20, and shaped as indicated in Fig. 2 so as topresent spaces between these fins to the opening which exists betweenthe lower end of the primary cathode l and the upper end of the tubularconduit The secondary cathode not only serves as a cathode member, butby surrounding the primary cathode i in the manner shown acts as a heatshield for the primary cathode d and prevents excessive loss of heatfrom said primary cathode. The fins likewise act as heat shields foreach other, prevent exces= sive loss of heat.

members it and 222 are electrically and mechanically connected togetherby means of straps ad which may be riveted to the upper end or theconduit 22 and the lower end or tubular member l l. The conduit 22 whichconducts from the pool to the interior of the cathode structure, isprovided with heat-insulatlng arrangement around its lower end. Thisheat-insulating arrangement consists of a tubular shielding membe 2bsurrounding and spaced from the conduit The lower ends of the shieldingmember and the conduit are mechanically connected by means oi an annularmember 2? which closes the space between the shielding member and theconduit, and prevents the material or the pool from rising into thespace between said members. in this manner an evacuated space existsbetween the conduit 22 and the shielding member 26, which effectivelyprevents the loss of excessive heat across such a space. The cathodestructure is supported in place on the stem 6 by means of a clampingstrap 2a which is clamped around the stem 6 and carries two supportingarms welded at their lower ends to the clamping strap at their upperends to the lower part or the heat shield 25. The stem ii has formed onone side thereof a glass bead Ell which engages with an aperture in theclamping strap 25, and efiectii' ely holds the clamping strap 2& inplace in the stem 8. The upper end of the extension l is provided with acup-shaped enlargement and the entire extension ll and most of thecup-shaped enlargement 32 are filled with the pool 25 of some easilyvaporizable material for supplying an atmosphere oi ionizable vapor tothe envelope E. The use oi mercury as the vapor-supplying rnaterial ispreferred. The enlargement 32 permits relatively large amounts ofmercury to m evaporated with a relatively small change in the level ofthe mercury pool. It will be seen that the mercury pool extends partlyup inside the conduit 22 and also around the heat-shielding member 2d,but does not extend into the space it. In order to steady the cathodestructure within the extension l, the heat shielding member 26 may beprovided with a number of leaf springs 33 fastened at their lower end tothe end of the heat shield 26. in placing the cathode structure withinthe extension l, these leaf springs engage the sides of the extension land resiliently resist lateral motion of the entire cathode structurewithin the extension i. In order to prevent excessive radiation of heatfrom the secondary cathode member onto the lower end wall of theenvelope l as well as the upper portion of the mercury pool, anadditional heat shield 3d is fastened to the conduit 22 and spaced fromthe lower side of the cap 2i. In order further to shield the upper sideof the mercury pool 25 from receiving excessive amounts of heat from thestructures above it, the upper end of the heat shield 26 is providedwith a shielding plate 35 which Fill projects over the upper surface ofthe mercury P001 25.

Cathode metallic surfaces will emit large numbers of electrons atcomparatively low temperatures ii an activating agent, such as alkali oralkaline earth metal, is deposited upon these surfaces. Since both ofthese metals are useful, we will use the term alkaline metal" hereafterin the specification and claims to cover both alkali metal and alkalineearth metal. In the particular embodiment described, barium in metallicform is preferably used as the activating agent. During operation,barium so deposited would be driven oh the surface of the cathode in acomparatively short time. This is due to the fact thatat the temperatureof electron emission, there considerable vaporization of barium and alsothe cathode may be subjected to positive ion bombardment which tends todislodge any layer of barium on the cathode. This difliculty isovercome, however, by continuously depositing upon the active cathodesurfaces metallic barium so as to renew the supply of metallic bariumthereon.

In order to supply the requisite amount of barium in the device, anannular cup constructed of a Washer 35, an inner sleeve 3?, and an outercylinder 38, all made of nickel, is mounted within the conduit 22. Themembers 36, 3? and 38 are secured together, as, for example, by welding,and

r the assembly is retained within the conduit 22 by means of asupporting wire welded at one end to the conduit 22 and at the other endto the cylinder 31. Within this annular cup a washer of barium isplaced, and retained in place by an upper washer 4|, also preferablymade of nickel. The central opening in this washer is larger than thesleeve 31 so that mercury may enter the annular cup, amalgamate with thebarium 40, and the amalgam so formed can pass out into the larger bodyof mercury within the lower portion of the tube. By thus forming anamalgam of mercury and barium, barium will be continuously conveyed tothe cathode surfaces during operation, as will be explained below.

During the processing involved in the manufacture of the tube, it isdesirable to have a sufficient amount of barium initially on certain ofthe cathode surfaces. This is preferably accomplished by first heatingthe pool 25, whereby sufficient material is vaporized from said pool tocarry enough barium up onto the cathode to enable easy initial startingto occur. Instead of supplying the barium in this way, a tab 42 may bemounted within the cathode chamber 13, which tab is wrapped around asmall amount of barium and supported within said chamber by means of anarm 43. The tab and arm being within the portion of the cathode whichreaches a fairly high temperature during operation are preferably formedof a refractory material, such as described above. Instead of using thebarium within the tab 42 for the initial starting of the tube, a lightcoating of electron-emitting oxides may be placed on the tubular member9. When the tube is first started,'these oxides are soon disintegratedand removed from the cathode surface. The deleterious gaseous productsof this disintegration are pumped out, leaving no material within thetube which during operation disintegrates and liberates deleteriousgases. By the time the oxide coating on the member 9 is destroyed, theaction of the tube will have deposited sumcient barium on the cathode totake care of the subsequent starting and running of the tube. In mostcases, however,.both the barium within the tab 42 and the flexibleconnection.

initial light coating on the member 9 may be omitted, and the bariumsupplied to the cathode, as

conveyed to the mercury pool vaporlzes some of the mercury, and aids inthe operation of the tube. Since in handling the tube there is apossibility of the lower end of the lead-in conductor 8 being subjectedto shocks and thereby fracturing the seal, it is desirable to protectthe leadin conductor 8 so that the possibilities of such shocks beingtransmitted to said lead-in conductor are eliminated. In order to dothis, connection is made to the conductor 5, as shown most clearly inFigs. 2 and 3. Around the lower end of said lead-in conductor is clampeda number of flexible leaf copper strips 44 by means of a pair ofclamping blocks 45 and 66. These blocks press the ends of the copperstrips as into intimate contact with the lead-in conductor 8 by a numberof clamping screws 4?. Around the lower end of the tubular extension 21is wrapped a metallic screen 48 about which is clamped the tubularclamping member 49 by means of clamping bolts 49'. The lower end of thisclamping member 49 projects beyond the lower end of the tubular member'5 and is reinforced by a ring 58 welded to the lower end of theclamping member 49. The copper strips 44 are looped around the leadinconductor 8 in the manner shown in Fig. 3, and are in turn securelyfastened to the lower end of the clamping member 49 by passing betweenthe two clamping members 5|, which are clamped onto the copper strips 44by means of a number of bolts 52. The outer end of the bundle of copperstrips 44 is provided with an opening 53 so as to facilitate connectionto an external conductor. Any strain put upon the conductors 44 or themember 49 is transmitted to the comparatively heavy wall of the tubularextension I, and thus the seal is effectively protected.

At the upper end of the envelope is formed a reentrant stem 53 throughwhich are sealed the lead-in conductors 54 and 55 for the filamentcurrent supply conductors l8 and I9. The leadin conductors 54 and 55 arepreferably formed of tungsten or molybdenum, and the glass of the sealsurrounding them is preferably a borosilicate seal glass. To the outerends of the lead-in conductors 54 and 55, may be connected flexibleconductors 55 and 51. whereby external connections to said conductorsmay easily be made. The connection between the lead-in conductors 54 and55 and the current supply conductors l8 and I9 is preferably flexible inorder to relieve the lead-in conductors 54 and 55 from as much strain aspossible. Due to this flexible connection, the weight of the cathodestructure is supported by the lower portion of the cathode I, and anyunusual expansion between the walls of the glass envelope l and thecathode structure and its lead-in conductors is taken up by said Thisconnection is made as follows. To the lower ends of the lead-inconductors 54 and 55 are welded connection plates 58 and 59,respectively. Also to the upper 75 cathode structure.

. front plate 18.

. nector 63 interconnects the lead-in conductor 55 and the currentsupply conductor IS. The connectors 62 and 63 are sufliciently resilientso that excessive strains cannot be transmitted to the lead-inconductors 54 and 55. However, these connectors are sufficiently rigidso that some steadying action is exerted by them upon the This steadyingaction is increased to some degree by forming shoulders 64 at the lowerends of the current supply leads !8 and I9, which shoulders engage theupper surface of the insulating bushing H. In order to prevent adischarge occurring between the current supply conductors l8 and 69, wesurround these conductors by a number of insulating tubes 65. Thesetubes are made of some refractory insulating material, such as magnesia.It is not necessary to cover the lead-in conductors 54 and 55 or theconnectors 60 and 6| inasmuch as these members are sufficiently removedfrom the cathode structure so that their temperature is low enough toprevent a discharge from occurring between them at the comparatively lowvoltthe clamping member 84' around the cylinder 80 age applied thereto.

Each anode structure consists of an anode 66 which is made of graphitein the form of a flat plate. Inasmuch as the structure is very compact,these is comparatively little space for the anodes. The amount ofcurrent which each anode can safely carry depends upon the anode areaover which the current can be distributed and the maximum temperaturewhich the anode reaches. This temperature depends in turn upon theamount of anode material and the amount of heat which the anode canradiate. By making the anodes in the form of flat plates, they areprovided with the maximum amount of current-carrying and radiatingsurface for the amount of anode material available. The anode 66 issupported from an anode lead-in conductor 61 made preferably of tungstenand sealed through one end of a reentrant stem 68. The outer end of thelead-in conductor 61 is connected to a flexible conductor 92, wherebyexternal connections may be readily made to the anode. The connectionbetween the lead-in conductor 61 and the anode 6B is made through a pairof clamping connectors 69 which are welded at one end to the inner endof the lead-in conductor 67. A plurality of screws 18 securely fastenboth connectors 69 to opposite sides of the upper end of the anode 66.

In order to shield the anode 66, it is surrounded with a shieldingstructure ll. This shielding structure consists of a metal frame 12,built up of thin nickel members. This metal frame supports a graphitebox which completely surrounds the anode 66 except for one or moredischarge openings in one side of said box. This box consists of a solidback plate 13, a top plate It, bottom plate 15, end plates '16 and 11,and a Each one of these plates is formed of graphite. The front plate 18may be formed with a discharge opening 19 which permits the discharge topass from the anode 66 to the cathode. Thus it will be seen that everysurface of the shield exposed to the anode 66 is composed of graphite.Graphite absorbs radiated heat readily, and in turn rapidly re-radiatesthe heat. Thus the graphite shield enables the heat generated at theanode to be readily dissipated. Graphite at the temperature at which theshields operate has practically no electron emission, which if itoccurred might adversely affect the shielding action. The shieldingstructure H is supported from the reentrant stem 68.

Around said reentrant stemis placed a cylinder 80 formed of somesuitable metal gauze, preferably nickel. This cylinder is closed at oneend by a plate 8| welded thereto, said plate having a relativelysmallopening'82 through which the conductor 61 passes. The plate 8| issecurely fastened to the metal frame 16 of the shielding structure H,as, for example, by being welded thereto. The cylinder 80 being formedof metal gauze does not have in itself sufiicient rigidity tosatisfactorily support the shielding structure ll. Therefore, a stay.member 83 welded at its lower end to a portion of the metal frame 16 andat its upper end to the cylinder 80, is used. The cylinder 80 and stay83 are firmly clamped around the reentrant'stem 68 by means of theclamping member 84. This clamping member is made of a nickel steel alloyhaving a coeflicient of expansion which is not greater than that of theglass in the stem 68. This insures that the clamp cannot loosen on thestem 68 during operation of the tube. Clamping bolts 85 areused totighten and stay 83. The reentrant stem 68 is provided with severalintegral projections 86 which project through openings in the clampingmember 84, and prevent any relative rotation between the shieldingstructure H and the reentrant stem 68. In order to increase the rigidityof the metal gauze cylinder 80, there is welded across the uppersidethereof two nickel strips 81 formed into an X-shape.

One end of said X is firmly retained in place under the clamping member84, and the other end thereof is welded to the cylinder 80 onto theplate 8|.

In the seal between the conductor 61 and the reentrant stem 68, theglass is preferably a borosilicate seal glass. The seal is protected bypacking the inside of the reentrant stem 68 with finely-powderedmetallic particles 90, which particles are held in place by retainingmembers 9|. This seal and the manner in which it operates is morecompletely described in the co-pending application of James D. Le Van,Serial No. 589,180, owned by the assignee of the present application. Inorder toprotect the seal against damage, it is desirable to keep it ascool as possible. For this reason the cylinder 88 is made of metal gauzein order to allow any heat liberated near the seal to readily escapetherefrom. Also any mercury vapor which enters the space within thecylindcr 80 is allowed to escape therefrom without accumulating therein.In this way there is very little or no tendency for a discharge to occurto the lead-in conductor 67 at its sealing point. To further protect theseal from overheating, heat generated at the anode 66 is preventedfrombeing radiated back onto the seal through the opening 82 by interposingbarrier members 88 and 89 between the anode and the seal. The barriermember 88 is a, ring fastened to the cylinder 80, and the barrier member89 is a plate fastened to the lead-in conductor 81. Each reentrant stem68 carrying its respective anode structure is mounted within anextension 93 formed integrally.with the glass envelope i. Thisprojection 93 is formed with a slanting lower wall 94, whereby mercurywhich condenses in this projection 93 runs back into the mercury pool.It will be noted that in the embodiment which is shown the rectifier isillustrated as being provided with six anodes. Thus the tube l is formedwith six projections 93 disposed about the central portion of theenvelope 1.

Each of the anode structures is disposed within the envelope I at anangle, as indicated in Figs. 9 and 10. The anode face opposite the open"ing 19 in the anode shield is directed downwardly toward the cathode.Thus each anode structure presents to its adjacent anode face only thesolid rear graphite plate of its shielding structure. The

only discharge path, therefore, which can exist between anodes is anindirect one. These shielding members effectively prevent dischargesbetween anodes despite the fact that the anodes are closely spacedwithin the envelope 5. Due to the position of each anode within thetube, any vapor which condenses on the walls of the envelope l and fallstherefrom cannot fail upon the face of the anode presented to thedischarge opening l9. The only anode structure surface upon which suchmaterial can fall is the solid graphite surface of the shieldingstructure H.

In order to provide sufilclent surface for condensing the vapors fromthe pool 25, the tube i is formed with a condensing chamber 95. Thiscondensing chamber is suiiiciently removed from the cathode structureand from the path of the discharge so that its temperature can bemaintained at any desired value and is substantially unailected by thetemperature of the region surrounding the cathode.

The tube which has been described may be connected in any suitablemanner to a rectifying circuit. For example, in Fig. i the tube is shownconnected to a six-phase rectifying circuit. The

secondaryof a transformer is represented at'96,

said secondary having six-phase windings. The primary of the transformer(not shown) may be connected in any suitable manner to a pluralphase A.0. power line. One of the anodes 66 is illustrated as being connected toone end of one of said phase windings. Each of the other phase windingsis in turn connected to each one of the other anodes. The neutral pointon said secondary 9B is connected through some suitable load 91 to thecathode lead-in connector 46. The heating filament to may be energizedby some suitable means, such as, for example, a filament transformer 98having a primary winding 89 connected to a source of alternatingcurrent, and the secondary source I connected to'the two lead-inconductors 54 and 55.

In constructing the novel tube, first of all the various elements areassembled, as shown in the drawings. Mercury is then introduced to thelevel indicated in Fig. 2. The exterior of the tubular extension I isthen heated and a considerable amount of mercury is vaporized whichcondenses upon the walls of the envelope I. This preliminaryvaporization of mercury is carried on so that when barium is releasedfrom the cathode structure and condenses upon the walls of the tube I,it will amalgamate with the mercury already there, which amalgam maypass readily back into the pool 25. Thus excessive deposits of bariumupon the glass walls are efiectively prevented. The mercury pool is alsoheated sufficiently to drive out from both the mercury pool and thebarium washer occluded gases. During this heating process a considerableamount, if not all, of the barium amalgamates with the mercury, thusleaving a mixture of mercury and barium in the pool 25. During thepreliminary treatment of the tube, an intense discharge is Z allowed topass to various elements within the tube, thus raising them to acomparatively high temperature and driving off all occluded gases.During this portion of the treatment, the tube is connected to anexhaust pump, and these gases are removed therefrom. After the tube hasbeen de-gasified in this manner, it will be noted that there are noelements which can decom- D se and liberate deleterious gases in thetube during operation theroi. This feature tends to produce a very longlife of the tube.

In accordance with the present understanding of the invention, thetheory of operation of the tube is as follows. When the filamenttransformer 5B is energized, a heating current flows in the heatingfilament l0, and quickly raises the tubular member 9 and the interior ofthe cathode chamber i3 to an operating temperature. During thepreliminary de-gasifying of the tube, sufilcient barium will have beenliberated either from the tab 52 or from the pool 25 so that the wallsof the cathode chamber is have deposited thereon a layer of barium. Atthe temperature to which the cathode chamber is is raised by the heatingfilament ill, the cathode surfaces carrying the barium emit largenumbers of electrons. As stated above, the heating time need merely belong enough to heat the cathode chamber l3 or the member 9 alone to thisemitting temperature. The secondary 95 may be energized at the same timeas the filament, and voltage is applied between the cathode and each ofthe anodes. The electrons emitted from the cathode chamber l3 passtoward any anode that is positive and ionize the mercury vapor withinthe cathode chamber l3 as well as in the space between the anode and thecathode. In this way an arc-like discharge is started between thecathode and the anodes. The heat liberated by this discharge and. alsothe heat liberated by the heating filament ac passes down both byradiation and by conduction through the conduit 22 to the mercury withinsaid conduit. This heat vaporices large quantities of mercury which passup through the conduit, and create a high density of mercury vaporWithin the cathode chamber This vaporized mercury passes out through theopening between the hollow member ii and the upper end of the conduit22, and creates a high density of vapor .l the fins within the secondarycathode chamber 29'. The mercury vapor then passes out into the envelopel and condenses on the walls thereof and upon the walls of thecondensing chamber 95. Thus low density of mercury vapor is adjacent theanode structures 3 and is determined largely by the temperature of thecondensing chamber 95 and the cooler portions of the walls of theenvelope i. This condition of high density of mercury vapor adjacent thecathode surface and low density of vapor adjacent the anode surfaces isconducive to effective rectifier action.

It has been found that while large quantities of mercury are beingvaporized from the pool 25, barium is carried up onto thecathode-emitting surfaces. It is believed that the mechanism wherebythe-barium is carried up onto the oath ode-emitting surfaces may consistof a number of phenomena. or a combination of two or more of thesephenomena. During the operation, the mercury within the conduit 22 maybe agitated rapidly, and due to the presence of the inner sleeve 31,whereby a restriction occurs within the conduit 22, percolation oi themixture of barium mercury amalgam within the pool 25 may take place; Dueto this percolating action, particles of barium mercury amalgam areforcibly thrown from the surface of the pool 25, and are carried by thevapor stream to the cathode surfaces. These particles not only depositupon the surface of the member 9 and inner surface of the tubular memberII, but pass out and are deposited onto the surfaces of the secondarycathode 5. The mercury may be vaporized at such a rate that the levelthereof falls below the surfaces of the members 36, 37 and 38. Thesemembers thereupon intercept the heat radiated and conducted from thecathode structure, and become quite hot. Mercury condensing in the tubeand returning to the pool 25 forces the amalgam to rise above the upperend of the cylinder 3! and drop onto these hot surfaces. This amalgam isthen heated rapidly and particles of barium mercury amalgam are thrownoff from these surfaces, thus causing barium mercury amalgam to bedeposited upon the cathode surfaces as described above. In addition tothe actions which have been described, the mercury vapor may come offfrom the surface of the pool 25 within the conduit 22 with sumcientrapidity to carry with it small mist particles of the amalgam. The rateat which the vapor travels up through the conduit 22 may be sufficientto carry these mist particles along with it and deposit them on thecathode surfaces. Since large quantities of mercury are evaporated fromand returned to the pool 25 and a small portion of barium may vaporizeat the same time and deposit on the oathode surfaces, this continuousaction may eventually deposit adequate quantities of barium on thecathode for proper operation. Thus it will be seen that during theoperation with the tube there is a continual deposit of barium ormercury barium amalgam upon the cathode surfaces. These surfaces aresufficiently hot so that the mercury of the amalgam is quickly vaporizedtherefrom, leaving residual barium. The temperature which the cathodesurfaces reach is sufficiently low so that barium can remain on thesesurfaces for a considerable period of time. The temperature is, however,sufliciently high so that the cathode surfaces carrying barium emitslarge amounts of electrons and enable the currents of large magnitudesto be drawn from the cathode. For example, cathodes have been madeapproximately the size as shown in the drawings from which over onethousand amperes have been drawn without adversely affecting thecathode.

The continual deposit of barium on the oathode surface renews theemitting surface of the cathode indefinitely. Italso permits heavyternporary overloads to be drawn from the cathode without adverselyaffecting either the operation or the life of the cathode. The normalvaporization of barium from the cathode surface and the dislocation ofbarium by positive ion bombardment during operation are more thancompensated for by the constant supply of barium to the cathode. Ifexcessive amounts of current are drawn for a short time from thecathode, much larger quantities of barium may be dislodged from thecathode surfaces. However, the barium so dislodged is immediatelyreplaced by a fresh deposit. Thus heavy surges of current which woulddenude an oxide-coated cathode of its active coating does not harm thepresent cathode. If, for example, an intense arc were to localize on thesurface of the cathode, the barium at the localized spot would be soondriven off. However, if this occurred, the voltage drop at this pointwould go up and the arc would have a tendency to spread to adjacentportions of the cathode having a layer of barium. The area denuded ofbarium by the localized arc would immediately receive a fresh deposit ofbarium. It will be seen, therefore, that the novel cathode which hasbeen devised is a thermionic cathode with a self-renewing surface.

The particular arrangement utilizing a vaporizable material, such asmercury, which is liquid at ordinary temperatures and an activatingagent, such as barium, which is solid at ordinary temperatures, performsan additional function. rdinarily if barium were placed upon a cathodesurface and driven ofi. due to the relatively high temperature of thecathode and the bombardment by positive ions, this barium would condenseupon the walls of the tube. At the usual temperatures at which tubes ofthis kind are operated, barium is solid. Therefore, in such anarrangement, the barium would collect upon the walls of the envelope andthere would be no way whereby such barium could be returned to thesurface of the cathode. However, by utilizing mercury which is liquid atthe usual temperatures at which such tubes are operated, the mercurywill amalgamate with the barium which tends to deposit on the walls ofthe tube, and this amalgam is sumo!- ently fluid so-that it runs backinto the pool 25. Thus the mercury serves not only to supply anionizable vapor in order to carry the discharge, but also acts as acarrier for the barium, whereby the barium is carried from the pool tothe cathode and then from the walls of the envelope.

back to the said pool.

As has been pointed out above, the discharge initially starts from theprimary cathode chamher is. This discharge passes out from the lower endof the tubular member H and past the surfaces of the secondary cathodeon its way to the anodes. This discharge quickly heats the surfaces ofthe secondary cathode to a high temperature so that the dischargeinstead of continuing within the primary cathode chamber l3 distributesitself over the surface of the fins within the secondary cathode chamber20'. It is generally considered that in any cathode there is certainmaximum density of current which can be drawn without increasing thecathode drop, and thus sufficient area of cathode-emitting surface mustbe provided in order to enable this maximum current density to exist atthe highest currents which may be drawn from the cathode. However, atsmaller values of load the current can safely be drawn from a smallercathode area. In the cathode described, the area which emits electronsautomatically adjusts itself so that the optimum current density existson the emitting surfaces of the cathode. For example, when a smalldischarge is drawn to the anodes, less heat is liberated at the cathodeand only a small portion of the secondary cathode 5 adjacent the primarycathode 4 is heated to its emitting temperature. cathode 5 emitselectrons, but the amount of electrons so emitted is sufficient to carrythe current. As more current is drawn from the cathode, more heat isliberated in the discharge, and a larger surface on the secondarycathode 5 is raised to its emitting temperature. Finally, at the maximumcurrent which is drawn from the cathode, the entire inner surfaces ofthe Thus only a portion of the secondary.

hollow secondary cathode are at emitting temperatures and the current isdistributed over all of these surfaces. As has been previously pointedout, the secondary cathode 5, due to the continual deposit of barium onit, hasa self-renewing emitting surface as well as the primary cathode4.

Tubes of the kind which have been described have a low voltage drop. Forexample, some tubes have beenconstructed in which the drop isconsistently eight volts and in some instances the drop is much less.Also the life of the tube is extremely long. Such tubes have beenconstructed which have been operating for over three thousand hourscontinuously and carrying hundreds of amperes of rectified current.

This invention is not limited to the particular details of construction,materials and processes described above. For example, instead of usingmercury vapor as the gaseous atmosphere within the tube, other vapors orgases could be utilized. Also the novel cathode shown herein could beutilized inpther types of devices, such as gaseous discharge lamps andthe like. Various other equivalents of these and other features of theinvention will suggest themselves to those skilled in the art. It isaccordingly desired that the appended claims be given a broadinterpretation commensurate with the scope of the invention within theart,

What is claimed is:

1. A gaseous discharge device comprising an envelope containingcooperating electrodes and an ionizable atmosphere, one of saidelectrodes being a cathode and another an anode, said cathode comprisinga primary cathode of solid metal, a heater for heating said primarycathode to temperature of thermionic emission, whereby an ionizingdischarge may be started between said primary cathode and said anode,and a hollow secondary cathode having an interior electron-emittingsurface out of substantial heat transfer relation with respect to saidprimary cathode heater disposed immediately adjacent the discharge pathbetween said primary cathode and said anode, whereby said, secondarycathode interior surface is heated by the discharge to temperature ofthermionic emission, said primary cathode having as an element thereof ashielding member shielding the emitting surface of said primary cathodefrom the emitting surface of said secondary cathode, whereby heattransfer from said primary cathode surface to said secondary cathodesurface is inhibited, said secondary cathode also having a dischargeopening whereby the discharge may pass to said anode.

2. A gaseous discharge device comprising an envelope containingcooperating electrodes and an ionizable atmosphere, one of saidelectrodes being a cathode and another an anode, said cathode comprisinga primary cathode of solid metal, a heater for heating said primarycathode to temperature of thermionic emission, whereby an ionizingdischarge may be started between said primary cathode and said anode,and a secondary cathode out of substantial heat transfer relationshipwith respect to said primary cathode and having an extended surface withone part thereof disposed immediately adjacent the primary cathode andthe discharge path between said primary cathode and said anode,adjoining areas of said extended surface being spaced progressivelygreater distances from said primary cathode, whereby said dischargeheats successively greater areas of said secondary cathode t0temperature of electron emission as the amount of current in thedischarge increases.

3. A gaseous discharge device comprising an envelope containingcooperating electrodes and an ionizable atmosphere, one of saidelectrodes being a cathode and another an anode, said cathode comprisinga primary cathode, a heater for heating said primary cathode totemperature of thermionic emission, whereby an ionizing discharge may bestarted between said primary cathode and said anode, and a secondarycathode out of substantial heat transfer relationship with respect tosaid primary cathode and disposed adjacent the discharge path betweensaid primary cathode and said anode, whereby at least a portion of saidsecondary cathode is heated by the discharge to temperature of electronemission, said secondary cathode comprising a solid member having anextended surface, said surface composed of a material having arelatively great thermal conductivity and emitting large numbers ofelectrons at the temperature to which .it is raised by the discharge,whereby as the amount of current in said discharge is increased,adjacent areas of said surface are heated to thermionic emission and thedischarge distributes itself over said surface.

4. A space discharge device comprising an envelope containingcooperating electrodes, one of said electrodes being a solid cathode andanother an anode, a lead-in conductor for said cathode sealed through awall of said envelope,

and a pool of a metal which is liquid at operating temperatures withinsaid container, said pool communicating with the space in which saidelectrodes are located for furnishing a vapor to said space, saidlead-in conductor being in intimate contact with said pool, said cathodebeing in intimate contact with said pool at another point, wherebyelectrical connection between said cathode and lead-in conductor isestablished through said pool.

5. A space discharge device comprising an envelope containingcooperating electrodes, one

of said electrodes being a solid cathode and another an anode, a lead-inconductor for said cathode sealed through a wall of said envelope, and apocl of mercury, said pool communicating with the space in which saidelectrodes are located for furnishing a vapor to said space, saidlead-in conductor being in intimate-contact with said pool, said cathodebeing in intimate contact with said pool at another point, wherebyelectrical connection between said cathode and leadin conductor isestablished through said pool.

6. A space discharge device having an envelope with a glass wall,electrodes within said envelope, 9. lead-in conductor for one of saidelectrodes sealed through said glass wall, a clamping member secured tothe exterior of said envelope adjacent said lead-in conductor, aflexible conductor having one end secured to said lead-in conductor andthe other end to said clamping member, and means for establishingelectrical connection to said flexible conductor at a point beyond saidclamping member on the other side thereof from said lead-in conductor. I

7. A space discharge device having an envelope with a glass wall, anelectrode Within said envelope, a lead-in conductor for said electrodesealed through said glass wall, a flexible conductor having one endsecured to said lead-in conductor and the other end secured to theenvelope, and means for establishing electrical connection to saidflexible conductor at a point beyond where it is secured to saidenvelope on the other side thereof from said lead-in conductor, and anadditional electrode cooperating with said first electrode. 4

8. A space discharge device having an envelope comprising a glass bodythrough which a lead-in conductor is adapted to be sealed, an electrodewithin said envelope, a lead-in conductor for said electrode sealedthrough said glass body, a flexible conductor having one end secured tosaid lead-in conductor and a point beyond said end secured to theenvelope, and means electrically connected to said flexible conductor ata point beyond where it is secured to said envelope on the other sidethereof from said lead-in conductor, and an additional electrodecooperating with said first electrode.

9. A gaseous discharge device comprising an envelope containing anionizable atmosphere, a cathode, and a plurality of anodes disposedaround said cathode, each of said anodes being surrounded with a shield,each shield having a discharge opening whereby a discharge may pass fromsaid cathode to the anode, the discharge openings in said shields eachfacing a solid wall of the adjacent anode shield.

10. A gaseous discharge device comprising an envelope containing anionizable atmosphere, a cathode, and a plurality of anodes disposedaround said cathode with their central points in substantially a commonplane, each of said anodes comprising a fiat conducting plate disposedat an angle to said common plane and at an angle with respect to anyplane at right angles to said common plane.

11. A gaseous discharge device comprising an envelope containingcooperating electrodes and an ionizable atmosphere, a cathode, and aplurality of anodes disposed around said cathode with their centralpoints in substantially a common horizontal plane, each of said anodescomprising a flat conducting plate disposed at an angle to said commonplane and at an angle with respect to any plane at right angles to saidcommon plane, each of said anodes being surrounded with a shield, eachshield having a discharge opening whereby a discharge may pass from saidcathode to the anode, the discharge openings in said shields facingdownwardly and a solid wall of each anode shield facing upwardly.

12. A gaseous discharge device comprising an envelope containingcooperating electrodes and an. ionizable atmosphere, one or saidelectrodes being a cathode and another an anode, a shield surroundingsaid anode, a lead-in conductor sealed through a wall of said envelope,and a perforated shield surrounding said lead-in conductor within saidenvelope and connected to i said first-named shield.

13. A gaseous discharge device comprising an envelope containingcooperating electrodes and an ionizable atmosphere, one of saidelectrodes being a cathode and another an anode, a shield surroundingsaid anode, a lead-in conductor sealed through a wall of said envelope,and a perforated shield surrounding said lead-in conductor within saidenvelope.

14. A gaseous discharge device comprising an envelope containingcooperating electrodes and an ionizable atmosphere, one of saidelectrodes being a cathode and another an anode, a shield surroundingsaid anode, a reentrant stem in said envelope, a lead-in conductorscaled through said stem, a perforated shield surrounding said lead-inconductor within said envelope and connected' to said first-namedshield, said perforated shield being fastened onto said stem, and arigid stay fastened at one end to said stem and at the other end to saidfirst-named shield, whereby both of said shields are firmly supported bysaid stem.

15. A gaseous discharge device comprising an envelope containingcooperating electrodes and an ionizable atmosphere, one of saidelectrodes being a cathode and another an anode, a shield surroundingsaid anode, a reentrant stem in said envelope, a lead-in conductorsealed through said stem, a perforated shield surrounding said lead-inconductor within said envelope and connected to said first-named shield,said perforated shield being fastened onto said stem, a rigid stayfastened at one end to said stem and at the other end to saidfirst-named shield, whereby both of said shields are firmly supported bysaid stem,

and reinforcing strips on one side of said perforated shield forassisting in said support.

16. A gaseous discharge device comprising an envelope containingcooperating electrodes and an ionizable atmosphere, one of saidelectrodes being a cathode and another an anode, a shield nectedrespectively to said perforated shield and lead-in conductor and spacedapart a sumciently short distance to prevent a discharge from passing tosaid anode through said perforated shield.-

17. A gaseous discharge device comprising an envelcm containingcooperating electrodes and an ionizable atmosphere, one of saidelectrodes being a cathode and another an anode, a shield surroundingsaid anode, a lead-in conductor sealed through a wall of said envelope,a perforated shield surrounding said lead-in conductor within saidenvelope and connected to said firstnamed shield, and two shieldingmembers connected respectively to said perforated shield and lead-inconductor and spaced a short distance apart.

18. A space discharge device comprising a sealed envelope containing ananode and a cathode having a discharge surface, said cathode dischargesurface being composed of a bare solid metal, a mixture of a vaporizablematerial for furnishing an ionizable vapor for supporting a dischargebetween said cathode and anode and a material which enhances andmaintains the electron-emitting qualities of said discharge surface whenconveyed thereto, and means for continuously conveying said mixture tosaid discharge surface, during operation.

19. In a gaseous discharge device comprising a sealed envelopecontaining an anode and a cathode having a discharge surface, saiddischarge surface being composed of a bare solid metal, the

method of constantly renewing the electron emitting properties of saidcathode which comprises continuously conveying to said discharge surfacethe vapor of an easily vaporizable material, said vapor constituting anionizable atmosphere within said envelope and causing said vapor soconveyed'to said discharge surface to carry along with it an activatingmaterial which enhances and maintains the electron-emitting qualities ofsaid discharge surface.

20. A space discharge device comprising a sealed envelope containing ananode and a cathode having a discharge surface, said discharge surfacebeing composed of a bare solid metal, a mixture of a vaporizablematerial for furnishing an ionizable vapor for supporting a dischargebe-- tween said cathode and anode and an alkaline metal which enhancesand maintains the electron-emitting qualities of said discharge surfacewhen conveyed thereto, and means for continuously conveying said mixtureto said discharge 10 surface during operation.

21. In a gaseous discharge device comprising a sealed envelopecontaining an anode and a cathode having a discharge surface, saiddischarge surface being composed of a bare solid metal,

II the method of constantly renewing the electronemitting properties ofsaid discharge surface which comprises conveying to said dischargesurface the vapor of an easily-vaporizable material, said vaporconstituting an ionizable atmosphere within said envelope and causingsaid vapor so conveyed to said discharge surface to carry along with itan alkaline metal which enhances and maintains the electron-emittingqualities of said discharge surface.

' 22. A gaseous discharge device comprising an envelope containingcooperating electrodes and an ionizable atmosphere, one of saidelectrodes being a cathode and another an anode, said cathode comprisinga primary cathode of 'solid metal Q having a surface adapted to beheated to thermionic emission, a heater for heating said primary cathodesurface to temperature of thermionic emission, whereby an ionizingdischarge may be started between said primary cathode and I said anode,and a secondary cathode of solid metal out of substantial heat transferrelation with respect to said primary cathode heater and having asurface disposed immediately adjacent the discharge path between saidprimary cathode o and said anode, whereby said secondary cathode isheated by the discharge, said surface being a coated with athermionically-active material during operation and being adapted toemit large numbers of electrons. thermionically at the temperature towhich it is raised by said discharge.

23. A gaseous discharge device comprising an envelope containing anionizable atmosphere, a cathode. and a plurality of anodes, each of saidanodes being surrounded with a shield, each shield having a dischargeopening whereby a discharge may pass from said cathode to the anode. thewalls of said shield interrupting all straightline discharge pathsbetween adjacent anodes by having the discharge openings in said-shieldseach facing a wall of the adjacent anode shield.

24. A space discharge device comprising a sealed envelope containing ananode and a cathode having a discharge surface, said cathode dischargesurface being composed of a bare solid 5 metal, a mixture of avaporizable material for furnishing an ionizable vapor for supporting adischarge between said cathode and anode and a material which enhancesand maintains the electron-emitting qualities of said discharge sur- 10face when conveyed thereto, and means for conveying said mixture to saiddischarge surface during operation.

25. In a gaseous discharge device comprising a sealed envelopecontaining an anode and a cathode having a discharge surface, saiddischarge surface being composed of a bare solid metal, the method ofrenewing the electron-emitting properties of said cathode whichcomprises conveying to said discharge surface the vapor of an easily 2ovaporizable material, said vapor constituting an ionizable atmospherewithin said envelope and causing said vapor so conveyed to saiddischarge surface to carry along with it an activating material whichenhances and maintains the electron-emitting qualities of said dischargesurface.

26. A space discharge device comprising a sealed envelope containing ananode and a cathode having a discharge surface, said discharge surfacebeing composed of a bare solid metal, a mixture of a vaporimble materialfor furnishing an ionizable vapor for supporting a discharge betweensaid cathode and anode and an alkaline metal which enhances andmaintains the electron-emitting qualities of said discharge surface whenconveyed thereto, and means for conveying said mixture to said dischargesurface during operation.

27. In a gaseous discharge device comprising a sealed envelopecontaining an anode and a catha ode having a discharge surface, saiddischarge surface being composed of a bare solid metal. the method ofrenewing the electron-emitting properties of said discharge surfacewhich cornprises conveying to said discharge surface the a vapor of aneaslJy-vaporiz'able material, said vapor constituting an ionizableatmosphere within said envelope and causing said vapor so conveyed tosaid discharge surface to carry along with it an alkaline metal whichenhances and maintains the electron-emitting qualities of said dischargesurface.

PERCY L. SPENCER.

LAURENCE K. MARSHALL

